1. Field of the Invention
This invention relates to a dryer for granular solid materials and more particular to a continuous dryer for minerals and ores which employs a single screw transport in a drying chamber with a plurality of external electrical heating elements surrounding the drying chamber.
2. Description of Related Art
As used in this invention the term "solids" will mean any solid broken up material, including but not limited to, coarse to very fine granular material such as sand of different textures, ores and minerals. The term "wet solids" will be used to indicate a mixture of solids with a volatile fluid, which may be but is not necessary limited to water, wherein the solids are not in solution in said fluid to any appreciable extend.
Dryers for removing a fluid from wet solids through the application of heat are well known in the art. Such dryers may be classified in two broad categories, batch dryers and continuous dryers. Batch dryers are dryers which comprise a drying chamber which is loaded with the wet solids, and the solids are heated in the chamber for a time sufficient to remove the volatile fluid therefrom, after which time the chamber is emptied and the process is repeated with a new load.
Continuous dryers on the other hand, as the name implies, operate by transporting in a continuous manner the solids to be dried through a drying zone. Fresh, wet solids are constantly added at one point in the dryer and dry material removed at another. In between, the solids are conveyed through a drying zone where heat is applied to remove the volatile fluids therefrom.
Continuous dryers require means to transport the wet solids through the drying zone. Screw conveyors are one known such means. To dry the material, screw conveyors may transport the material in contact with a current of a flowing heated medium, typically heated air. The flow of the heated medium may be in the direction of the transport of the solids, or against it. Such arrangement, while efficient in removing a volatile fluid from the drying material, is disadvantageous when used for drying fine solids, because the flowing heated medium produces a substantial amount of dust particles which must usually be contained and removed from the environment.
In an effort to prevent this problem it is known to use a hollow conveyor screw and pass the heating medium through the body of the conveying screw thereby heating the transported material without contacting the material and thus minimizing the dust generation. In both instances however there remains the problem of efficiently heating the ore or mineral in a more or less uniform manner in order to completely dry all of the material. It has been the experience of the art practitioners that wet ores, minerals and other such granular materials tend to agglomerate and compact during transport particularly when screw conveyors are used, producing agglomerations of material which is not completely dry. In addition, solids tend to stick and form a cake on the shaft and flights of the conveyor screws and on the inner wall of the dryer, reducing heat transfer efficiency.
Another problem encountered in continuous dryers particularly where the heating fluid is directed through the screw transport body, relates to the bearings supporting the screw which are subjected to high heat, requiring the use of special materials both for the bearings and for the lubricant used in the bearings.
U.S. Pat. No. 4,430,057 teaches a dryer for driving volatile substances from materials in a horizontal drying chamber which appears to address some of the prior art problems. To avoid the agglomeration problem, this reference teaches the use of a screw conveyor wherein the screw flights incorporate scrapers and lifters. A plurality of parallel, interacting, screw conveyors is suggested for optimum efficiency. Heat is applied externally of the drying chamber using a plurality of fuel burners to heat the upper portion of the drying chamber. The screw conveyor is located at the bottom end of the drying chamber and the agitating paddles on the conveyor serve to constantly bring fresh material to the upper, hotter side of the chamber while transporting the solids from an input end of the dryer chamber to the other. During drying and transport the treated material remains at the lower part of the heating chamber.
While this dryer structure represents an improvement over prior dryers, there remains the problem of dust generation in the space above the conveyor screw as a result of the agitation. This dust is not only a nuisance, but can be dangerous when pyrotechnic or heat sensitive ores are processed, because of the presence of the open flame used to heat the drying chamber. Further more, in the disclosed structure heat is applied to the ore primarily by radiation from above, supplemented with any incidental convection at the lower, unheated portion of the drying chamber, the only place where the ore is in contact with the chamber wall. Finally, the multiplicity of conveyor screws specially designed to interact in agitating the ore results in a dryer that is complex and expensive to produce.
Thus there is still need for a safe, simple, continuous dryer which offers high heating efficiency, avoids agglomeration, caking and compacting problems, produces a minimum amount of dust and is safe and cheap to manufacture and operate.
These and other objects of the present invention will be clear from the following description.